Heat oil in a sauce pan on low heat.
In the meantime purée your tomatoes and garlic and set aside.
Roast your spices till fragrant and put in your mortar and pestle and crush
Now increase the heat to medium high and add the mustard seeds, dried chilies and curry leaves.
When it starts sputtering add the onions and potatoes and spices.
Stir for a few minutes and then add the puréed tomatoes and garlic.
Cover and cook on slow to medium heat till done.
Garnish with fresh coriander.